1. Field of the Invention
The present invention relates to a measuring instrument that measures a swing movement of a swinging body when a probe of the swinging body touches a workpiece with a predetermined measuring force.
2. Description of Related Art
Conventionally, measuring instruments for measuring a surface texture such as roughness, waviness, contour and roundness of a workpiece, e.g. surface roughness measuring instrument, contour measuring instrument and roundness measuring instrument, have been known. In such measuring instruments, a sensor scans the surface of the workpiece to collect scanning data, based on which the surface texture is obtained. The sensor includes a swingably supported arm having a probe to be in contact with the surface of the workpiece at a tip end thereof. In the conventional measuring instrument, the arm is swung according to the movement (displacement) of the probe in accordance with the irregularities on the workpiece surface and the swing amount is detected as scanning data.
A measuring instrument shown in Japanese Utility Model Registration No. 2556550 (P. 2, right column to P. 4, right column) has an arm swingably supported at a fulcrum. A probe to be in contact with and movable along a surface of a workpiece is provided on an end of the arm. On the other end of the arm, a weight and a motor for moving the weight are provided. A balance controller controls the motor to move the weight so that the probe touches the workpiece with a predetermined measuring force to balance the arm, where the probe is moved along the surface of the workpiece and the movement of the arm displacing together with the probe is measured, thereby measuring the surface texture of the workpiece.
However, according to the measuring instrument described in the Japanese Utility Model Registration No. 2556550, the inertia of the entire arm is increased on account of the motor provided on the arm, so that the frequency characteristics of the swinging probe may be deteriorated, which may result in deterioration in the swingability of the arm. Accordingly, tracking response to minute irregularities on the workpiece surface may be deteriorated, and when the scanning speed for moving the probe along the workpiece surface, i.e. moving speed of the probe relative to the workpiece surface is accelerated, measurement accuracy may be deteriorated. Therefore, it is difficult to increase the scanning speed and the measurement efficiency may not be improved. Further, since the the entire arm bears great inertia, when the probe is scanned along the workpiece surface, excessive measuring force in the scanning direction (i.e. pressing force of the probe against the workpiece in the scanning direction) may be applied, so that the workpiece surface and the probe may be damaged.
A measuring instrument shown in Japanese Patent Laid-Open Publication No. 05-340706 (P.3, left and right columns) also has an arm swingably supported at a fulcrum. A probe to be in contact with and movable along a surface of a workpiece is provided on an end of the arm. A spring that applies a biasing force for swingably moving the arm in a direction opposite to the direction for the probe to be into contact with the workpiece surface is provided on an end of the arm. On the other hand, an iron core projecting in a swing direction of the arm is provided on the other end of the arm. Further, a first coil is provided on the other end of the arm. A second coil is provided on the measuring instrument so that the iron core of the arm is capable of moving substantially in the axial direction of the central axis of the second coil. The measuring instrument further has a cylindrical magnetic component of a voice coil motor in which the first coil is capable of moving in the axial direction thereof. The probe is moved along the workpiece surface and the movement of the arm integrally displacing with the probe is read based on the current value from the second coil in which the iron core is relatively moved. The voice coil motor is actuated by passing an electric current to the first coil in accordance with the current value from the second coil so that the probe is in contact with the workpiece surface with a constant measuring force by generating a reaction force against the biasing force of the spring.
However, according to the measuring instrument disclosed in the Japanese Patent Laid-Open Publication No. 05-340706, in order to control the measuring force of the probe against the workpiece, wirings such as a lead wire for applying the electric current to the first coil of the swinging arm is necessary. Accordingly, a stress on account of the rigidity of the wiring is applied on the swinging arm as a disturbance while swingably moving the arm, thereby fluctuating the measuring force. Accordingly, a complicated wiring without applying disturbance on the arm is required, so that it is difficult to improve the productivity. Further, desired measuring force may not be obtained on account of the snapping of the wire caused by repeatedly applying a load on the wiring by the swinging movement of the arm. Further, since the arm swingably moves, the coil of the arm moves on an arc in the voice coil motor. In order to improve the accuracy of the measurement, the coil of the arm is required to linearly move in the axial direction relative to the magnetic component, however, it is difficult to convert the arc movement of the coil of the arm into a linear movement solely by the arrangement of the voice coil motor. Accordingly, the arc movement has to be corrected as a linear movement, which requires an additional circuitry for the linear conversion, so that the structure of the instrument becomes complicated and productivity improvement and cost reduction are difficult to be achieved.
A measuring instrument shown in Japanese Patent Laid-Open Publication No. 2000-111334 (P.3, right column to P.5, right column) also has an arm swingably supported at a fulcrum. A probe to be in contact with and movable along a surface of a workpiece is provided on an end of the arm. A movement sensor for detecting the swing movement of the arm is provided on the other end of the arm. A wire etc. is connected to the other end of the arm. A measuring force adjuster for adjusting a measuring force to bring the probe into contact with the workpiece by applying a tension on the wire along the swinging direction of the arm by a motor and the like is provided on the measuring instrument. The arm of the measuring instrument is capable of being rotated so that the angle between the probe relative to the horizontal surface and a direction for the probe to be in contact with the workpiece, i.e. the angle of the arm relative to the longitudinal horizontal surface of the arm can be changed. A command value for determining the drive condition of the measuring force adjuster so that an appropriate measuring force in accordance with the combination of the type of the probe, the inclination angle of the sensor (i.e. the turn angle of the arm) and the like is applied is inputted in advance. In the measuring device, the type of the probe is inputted and the inclination angle of the sensor during measurement is detected to read the corresponding command value in accordance with the detected result, so that the drive condition of the measuring force adjuster is controlled to keep a constant measuring force in measuring the surface texture of the workpiece.
However, in the measuring device disclosed in the Japanese Patent Laid-Open Publication No. 2000-111334, a multiple of factors such as the inclination angle and the type of the probe influence on the measuring force and more number of the command value in accordance with the combination of the factors are required. Accordingly, complicated process is required for calculating the command value for achieving a constant measuring force in any of the combinations and for inputting the multiple number of the command values, so that the improvement in the productivity is required.
As described above, in the measuring instrument disclosed in the Japanese Utility Model Registration No. 2556550, the inertia of the arm is increased on account of the presence of the motor for moving the weight for keeping the constant measuring force and the tracking response of the probe relative to the irregularities on the workpiece surface is deteriorated, so that the measurement efficiency cannot be improved on account of the difficulty in increasing the scanning speed of the probe and the workpiece surface may be damaged by the probe. In the measuring device disclosed in the Japanese Patent Laid-Open Publication No. 05-340706, the wiring of the voice coil motor for keeping the constant measuring force may influence on the swing movement of the arm and the design without being influenced by the wiring is difficult. Further, a circuitry for converting the arc movement in the voice coil motor into a linear movement is required and the productivity may not be improved. In the measuring instrument disclosed in the Japanese Patent Laid-Open Publication No. 2000-111334, it is difficult to calculate and input the command value for setting the driving condition of the measuring force adjuster for keeping a constant measuring force and improvement in the productivity is desired.